Directional drilling, and more particularly, horizontal directional drilling, is a well-known technology that is used to form boreholes, typically for pipeline construction, although other applications are also known. In a typical pipeline construction application the directional drilling operation may be accomplished in three main stages. The first stage involves the drilling of a relatively small diameter pilot hole in the formation so that it follows a defined directional path established for the pipeline. The second stage, commonly referred to as a reaming stage, involves the use of a reamer to enlarge the size of the pilot hole to accommodate the desired pipeline. Depending the required final size of the borehole, several reaming steps may be required, with reamers of gradually increasing diameters being used to enlarge the borehole to the desired size. After the reaming stage, the pipeline may then be pulled back into the enlarged borehole to complete the process.
As mentioned, the pilot hole drilling apparatus is steerable or directable so that the pilot hole may be formed along the planned or desired pathway. Any of a wide range of steerable or directable drill strings and surveying techniques may be used for this purpose. While the pilot hole may follow the defined path within an acceptable tolerance, the subsequent reaming and pipe pulling operations may result in significant deviations from the path defined by the pilot hole, particularly if the pilot hole extends through formations of different types and properties.
For example, if the borehole traverses a rocky formation, it is possible that during the reaming process the borehole can ‘walk’ up to half the diameter of the final reamed size to get around a harder section of the rocky formation. In a sand or dirt hole, it is possible that a reamer can drop more than 3 meters from the path of the pilot hole. Both of these occurrences not only would place the pipeline in a different location than the desired pathway, but the undetected deviation may place added stress on the pipeline, thereby increasing the possibility of an in-service failure. Moreover, increasing constraints in pipeline development and the desire or necessity to place increasing numbers of pipelines in existing rights of way means that it is more important than ever to ensure that the installed pipeline does not deviate significantly from its planned path.